The present invention relates generally to spacecraft attitude control, and more particularly to increasing the roll offset operating range of a spacecraft.
Geosynchronous communications spacecraft typically use earth sensors to provide attitude information necessary for earth-pointing control. The advantage of using earth sensors is that they are simple and reliable devices that provide a direct measurement of the spacecraft attitude relative to the Earth. The drawback is that they have a limited sensing range, in part due to the small angular earth radius (8.7 degrees) at geosynchronous altitude. In particular, the scanning earth sensor used by most spacecraft manufacturers has a pitch range of about ±5.1 degrees, and a roll range of roughly ±2.4 degrees.
When a spacecraft is constructed, the earth sensor is mounted so that the sensor operates well within its range during normal mission operations. The difficulty occurs when a spacecraft already in-orbit is to be used for another mission. In this case, it may be necessary to tilt the spacecraft to move the antenna coverage pattern to a new geographical location. The limited roll sensing range, however, may make it impossible to achieve the required tilt. That is, tilting the spacecraft by the desired angle, may saturate the earth sensor so that it is inoperable at the new position. Thus, what is needed is a system and method that will allow the spacecraft and the earth sensor to operate at a roll offset.